Many rotationally-molded plastic products, such as suitcase halves or two piece, hollow shipping containers, require use of latches, hinges and occasionally handles. These items must necessarily be attached to portions of the container adjacent the parting line of each half of the container and preferably within grooves or indented portions of the container side wall. If the container is to be water-tight and/or air-tight, not only initially but over a period of years, the latches and hinges must be attached in a way that will be strong enough to withstand shock forces up to the strength of the attached items and so that they resist a steady force equal to that portion of the compressive force on the container's gasket that each item bears, as well as repetitive intermediate forces due to vibration.
Since attached items are subject to damage in service, it is also necessary to be able to replace them in the field with relatively simple equipment and without causing damage to the container. Fasteners can include such items as rivets, especially "blind" rivets, and screws with rivets being preferred as they are less likely to be removed by unauthorized persons trying to open the container during transportation and handling.
In known arrangements where hardware is directly attached by rivets to the plastic wall of the container, the plastic, such as polyethylene, may not be hard or strong enough to hold the rivet and resist forces that may be applied. Consequently, various strengthening approaches have been tried, such as using a metal washer, in an effort to strengthen the inner joint between the rivet and the inner surface of the container so the rivet will not pull out. These approaches have not proved to be entirely successful.
While rivets will normally be set tightly initially and will exhibit sufficient strength to hold a latch, for example, tightly against the container wall, and while there may be sufficient frictional force to help resist latch pull, such a connection will weaken in time due to rough handling during service in transportation where impact loads may have been applied to the latches and where there may have been alternate exposure to high and low temperatures. The impact loads will cause the rivets to loosen which, in turn, will change the latch connection. Also, it has been found that at somewhat elevated temperatures, the thermal expansion of the riveted plastic material immediately around the rivet and between the latch and the washer causes a thermal stress far beyond the compressive yield stress of polyethylene at the elevated temperature. Consequently, elevated temperatures cause permanent loosening of the rivets once the plastic returns to room temperature.
A widely used commercially available, latch, is the Simmons Link-Lock No. 2. When made of low carbon steel, this latch is rated at 350 lbs pull. It is capable of producing about 90 lbs. of gasket compressing force without undue pressure on the operator's fingers. This latch is normally attached with two 1/8 in. diameter rivets. If the plastic wall of the container is 0.20 in. thick, the average compressive stress in the plastic due to a 90 lb. steady load after the rivets are slightly loosened, is ##EQU1## The actual peak stress in the plastic material is substantially higher because the rivet, once it is not axially tight, bears more heavily on the inner and outer surfaces of the container. A steady compressive stress of over 300 lb/in.sup.2 causes creep in polyethylene even at room temperature. At higher temperatures, the polyethylene is even less able to support the stress. Also, at higher temperatures, expansion of the polyethylene in the pull direction of the latch, causes a greater steady force to be exerted on the latch which in turn increases the stress in the material around the rivets.
During container impacts, a latch rated at 350 lb. pull may exert forces up to this amount on the pair of 1/8 in. rivets. For a 300 lb. latch force, the average compressive stress around the rivet for 0.20 in. thick material would be: ##EQU2## This value is far beyond the yield stress of polyethylene.
During vibration in transportation, the container latches may be subjected to forces greater than the gasket force and less than impact forces. These intermediate forces may well be exerted thousands of times, and will gradually cause the rivets to become loose.
When a rivet becomes loose, the likelihood of air and moisture leaking in is greatly increased. Some container manufacturers daub sealant over the inner ends of the rivets in an effort to prevent such leakage.
If the rivets become sufficiently loose, the compression of the gasket material can be reduced and, thereby, permit leakage at the container parting line.
In an effort to reduce the deficiencies of direct-riveting latches and hinges to polyethylene containers a rotomolded container using embedded rivet inserts or receptors was developed and is described in Barstow, Jr. U.S. Pat. No. 4,284,202. Here the embedded rivet insert receptors are one piece, elongated oval structures substantially like the one shown in FIGS. 1-4. The substance of Barstow, Jr. is hereby incorporated by reference.
The insert 10, is comprised of a body portion 12 formed with front and rear sections, 14 and 16, respectively, integrally connected together by a hinge 18. A pair of relatively long out-board wings 20 and 22 were used with each insert and extended from opposite sides of the front section 14. Each wing included spaced apart upper and lower horizontal arms, 24 and 26 and an outer vertical arm 28 connecting the upper and lower arms together. As shown in FIGS. 1 and 4, just prior to use wings 20 and 22 would have to be bent at an angle from about 30 degrees to about 60 degrees toward the front section so as to conform to and generally follow the desired container shape where the insert was to be placed. Each insert was usually positioned in a well or recessed area, often called a groove, molded within the side wall of the container. Further, the insert was most desirably positioned adjacent or close to the parting line of each of the top and bottom sections of the container.
The front wall of section 14 of the insert was also provided with apertures 30, 32 through which blind rivets 34 could be inserted to fasten a latch or other item as indicated at 36.
During molding the insert shown in FIGS. 1-4 was temporarily fastened to the interior surface of the mold by a magnet. The front wall, however, extended only slightly ahead of the wings, in an attempt to space the wings outwardly away from the interior of the mold in order to allow plastic to be formed there about and the front face was generally rounded except for a flattened area between apertures 30 and 32. It was also hoped that the insert would provide a greatly increased bearing area within the molded container material as compared with only using two 1/8 in. rivets directly in the plastic. When the rear section 16 was folded into place via hinge 18 it provided a hollow space 38, behind the front section, for the expansion of blind rivets. This space was made deep enough, front-to-back, to permit insertion of unpulled blind rivets, and with a volume sufficient to accommodate several drilled-off rivet ends to provide for re-riveting should that have to be done.
The side wings or loops of the insert were designed to provide part of the bearing area and to resist the rotational forces exerted by the latches and hinges when closed. The wings were also designed to provide resistance to side-wise loads exerted by latches which might occur when the latch was open and hanging out from the container and struck from the side.
It was found that the dimensions of the molds themselves varied from one mold to another so that in order to accommodate the shape of the indent or groove portion where this insert was to be placed, wings 20 and 22 had to be bent manually and in different amounts in order to accommodate the particular mold in which an insert was to be used. In many instances, wings 20 and 22, which were substantially long and about equal to the length of the main body portion, would be bent incorrectly either too far forward toward the mold or not bent far enough so that they would project too far toward the rea surface what would become the inside of the finished container. If the former occurred, the wings themselves would protrude through or be visible at the front surface of the container and by reducing the distance between the mold and the wings flow of the powdered plastic resin about the wings as well as the front of the insert was impaired. This caused the creation of voids at various places about the wings and about the main body portion, relative to the front face of the insert, which were objectionable. If the latter occurred, the wings could protrude through to the interior of the container and not be fully embedded.
It was also found that in practice the wall thickness of the containers had to be 0.2 inches or greater to avoid having leaks due to molding voids about the inserts which communicated through the container wall. In many cases, the wall thickness of the container was greater than otherwise necessary due to the inserts.
In order to minimize container weight and cost, it is desirable to have an improved insert which can be molded in a leak-proof manner in thinner container walls while retaining satisfactory attachment strength.